Among automobile components, a steering rack bar (also simply referred to as “rack bar” hereinafter) used in a steering device is an important component serving as a backbone that changes the traveling direction of a car and connects right and left wheels, and if this steering rack bar is damaged, a steering wheel cannot be operated. Thus, a steel material used for a rack bar needs to be very reliable.
It should be noted that a rack bar has been conventionally manufactured by performing thermal refining, namely hardening and tempering, performing drawing processing as required thereafter, and performing punching and gear cutting through cutting processing on a round steel material obtained by rolling a medium-carbon steel material, and then performing induction hardening and tempering on the tooth-shaped portions. It should be noted that the rolled round steel material means a steel material whose cross-section is processed into a circular shape by rolling, and gear cutting means forming tooth-shaped portions.
In addition, it is required that a rack bar that has undergone the induction hardening does not break due to a crack in an induction hardening layer developing in a base material, even when an excessive load is applied thereto.
Furthermore, deep hole drilling is performed on the rack bar in the lengthwise direction of a central portion in the diameter direction.
Therefore, a round steel material used as the raw material for a rack bar needs to have good machinability and excellent base material impact properties (base material toughness) that resist crack development.
Inventors of the present invention proposed the following steel material, for example, as a steel material used in such a steering rack bar.
Patent Document 1 discloses a rolled steel material for induction hardening that contains, in mass %, C in an amount of 0.38 to 0.55%, Si in an amount of 1.0% or less, Mn in an amount of 0.20 to 2.0%, P in an amount of 0.020% or less, S in an amount of 0.10% or less, Cr in an amount of 0.10 to 2.0%, Al in an amount of 0.10% or less, N in an amount of 0.004 to 0.03%, the remaining portion being constituted by Fe and impurities, and has a chemical component in which the value of in 1 expressed by the equation [fn 1=C+(1/10)Si+(1/5)Mn+(5/22)Cr+1.65V−(5/7)S] (where C, Si, Mn, Cr, V, and S in the equation respectively express the amounts of the elements in mass %) is 1.20 or less, the microstructure being constituted by ferrite, lamellar pearlite, and spheroidal cementite, the average crystalline particle diameter of the ferrite being 10 μm or less, the area percentage of lamellar pearlite with a lamellar interval of 200 nm or less with respect to the microstructure of the lamellar pearlite being in a range of 20 to 50%, and the number of pieces of spheroidal cementite is 4×105 pieces/mm2 or more.
It should be noted that this rolled steel material for induction hardening may further contain at least one selected from Cu, Ni, Mo, Ti, Nb, and V.
Patent Document 2 discloses a rolled steel material for induction hardening that contains, in mass %, C in an amount of 0.38 to 0.55%, Si in an amount of 1.0% or less, Mn in an amount of 0.20 to 2.0%, P in an amount of 0.020% or less, S in an amount of 0.10% or less, Cr in an amount of 0.10 to 2.0%, Al in an amount of 0.010 to 0.10%, N in an amount of 0.004 to 0.03%, the remaining portion being constituted by Fe and impurities, and has a chemical component in which the value of Ceq expressed by the equation [Ceq=C+(1/10)Si+(1/5)Mn+(5/22)Cr+1.65V−(5/7)S] (where C, Si, Mn, Cr, V, and S in the equation respectively express the amounts of the elements in mass %) is 1.20 or less, the total content of Si, Mn, and Cr satisfying 1.2 to 3.5%, the microstructure being constituted by ferrite, lamellar pearlite, and spheroidal cementite, and the average crystalline particle diameter of the ferrite being 10 μm or less, the area percentage of lamellar pearlite with respect to the microstructure being in a range of 20% or less (including 0%), and the number of pieces of spheroidal cementite being 6×105 pieces/mm2 or more.
It should be noted that this rolled steel material for induction hardening may further contain at least one selected from Cu, Ni, Mo, Ti, Nb, and V.